FIG. 1 schematically shows a major portion of a conventional high power FET amplifier, such as a high power GaAs FET amplifier. A GaAs FET 4 is formed on a GaAs substrate 2, with its source electrode S being grounded. The gate electrode G of the FET 4 is connected to a quarter wavelength distributed constant circuit 81 through a bonding wire 6 shown as an inductance in FIG. 1. The quarter wavelength distributed constant circuit 81 is connected through another quarter wavelength distributed constant circuit 82 to an input terminal 10. The two quarter wavelength distributed constant circuits 81 and 82 form an input impedance matching circuit 13.
The drain electrode D of the FET 4 is connected through a bonding wire 12 to a quarter wavelength distributed constant circuit 91 which, in turn, is connected to an output terminal 14 through another quarter wavelength distributed constant circuits 91 and 92 form an output impedance matching circuit 23. In the conventional amplifier shown in FIG. 1, each of the input and output impedance matching circuits comprises two quarter wavelength distributed constant circuits, but the number of quarter wavelength distributed constant circuits is not limited to two. Any number of distributed constant circuits with a given electrical length and a given impedance can be used to provide a desired impedance.
In order for the conventional high power GaAs FET amplifier shown in FIG. 1 to be able to provide a large output power, it employs a GaAs FET having a very large total gate width. Thus, the input impedance of the GaAs FET 4 itself is very small. Furthermore, for forming a high power GaAs FET amplifier in a hybrid circuit configuration with an FET and other circuit elements, impedance matching must be done with overall S parameters of the circuit inclusive of the FET 4 and the bonding wires 6 and 12, taken into account. In general, as the operating frequency increases, the overall S parameters become more inductive than capacitive. As stated previously, in the high power GaAs FET amplifier, impedance matching for the circuit inclusive of the FET 4 and the bonding wires 6 and 12 is provided by the two quarter wavelength distributed constant circuits 81 and 82 in the input side and the two quarter wavelength distributed constant circuits 91 and 92 in the output side.
As described above, in a conventional high power GaAs FET amplifier, the impedance of a GaAs FET is very low, and impedance matching must be provided by means of distributed constant circuits under conditions where variations with frequency of the S parameters of the GaAs FET 4 are large. Accordingly, the frequency range in which matching can be achieved with a gain which can be considered to be constant is undesirably narrow. In addition, in the conventional high power GaAs amplifier, the return loss at the input or output side is large, which results in a smaller effective power that can be supplied to a load.
FIG. 2 is a graph showing calculations of input and output return loss versus frequency of the conventional high power GaAs FET amplifier of FIG. 1 with S parameters under small signal conditions. A curve S.sub.11 represents the return loss at the input and a curve S.sub.22 represents the return loss at the output. For instance, over an operating frequency range of from 5.0 GHz to 9.6 GHz, the return loss S.sub.22 is less than 6 dB, which is more or less satisfactory. However, the return loss S.sub.11 at the input is greater than 6 dB over the same frequency range and its maximum value is as large as 3 dB.
FIG. 3 shows the gain versus frequency characteristic of the high power GaAs FET amplifier of FIG. 1, which shows that the gain S.sub.21 is smaller than 8 dB and, in particular, it decreases to a value less than 5 dB at frequencies higher than 7 GHz. These values are not sufficient.
The present invention eliminates problems seen in conventional high power GaAs FET amplifiers such as the one described above. According to the present invention, a high power FET amplifier exhibiting a high gain over a frequency range wider than 1 octave is provided.